If you really must place blame:
If you had the wrong address on eBay and that is the one sent to the seller then the initial error is yours. But there is a difference between a wrong address and an address that is not the full one, so there's a little lack of clarity there. You say you had only supplied the postcode ?!?
If it really was only the postcode and she posted anyway then some blame transfers to the seller for not checking first. Unsure how much though.
Meanwhile, whilst I'd fully expect the carrier to make excuses, the very idea they accepted a parcel with just a postcode (which boils the possible destination down to what; a street maybe ?) and yet failed to find the intended recipient, but sat on their hands until the timeout period allowed them to return it: well I find that appalling. Either they should have made best effort to find out who at that postcode had the right name, or they ought not have accepted the parcel at the counter at all.
Still if your conscience is bothering you you can always offer. Your offer would probably be refused for the sake of customer relations anyway.